Close-packed monolayers of charged Janus-type nanoparticles at the air-water interface.

We present a new method to obtain close-packed monolayers composed of noble metal nanoparticles (NP) possessing well-defined permanent charge of either sign. The method is based on the fact that ligands forming the protecting layer exhibit ability to rearrange at the NPs surface. We demonstrate that if the protecting layer is composed of a mixture of hydrophobic and hydrophilic charged ligands in appropriate proportion, the NP exhibits properties of a Janus-type particle with one of the hemispheres hydrophilic and the other hydrophobic. Such amphiphilic NPs self-assemble into a monolayer of well defined surface charge at the air-water interface. Due to strong stabilizing effect of the lateral electrostatic repulsions, such monolayer can be compressed to form close-packed hexagonal structure, and then easily transferred onto a solid substrate with the Langmuir-Blodgett technique.